The present invention relates generally to a wide-angle zoom lens including a wide-angle end having a large angle of view. Specifically, the invention relates to a wide-angle zoom lens used with electronic imaging devices such as CCDs or CMOSs. More specifically, the invention relates to a wide-angle zoom lens well suited for digital single-lens reflex cameras having a reflective surface at a back focus, and an imaging apparatus incorporating the same.
In recent years, the market for single-lens reflex cameras using electronic imaging devices such as CCDs or C-MOSs has grown steadily. However, there have not been many proposals of a superwide-angle zoom lens that has an angle of view of 100° or greater at a wide-angle end, is of small-format size and comprises fewer lenses. For instance, Patent Publications 1, 2 and 3 have come up with a superwide-angle zoom lens having an angle of view of about 102° and a zoom ratio of about 1.9, a superwide-angle zoom lens having an angle of view of about 110° and a zoom ratio of about 1.7, and a superwide-angle zoom lens having an angle of view of about 110° and a zoom ratio of about 1.5, respectively.
Patent Publication 1: JP(A)2004-21223
Patent Publication 2: JP(A)2007-94176
Patent Publication 3: JP(A)2001-166206
For the arrangement of such superwide-angle zoom lenses, it is general that there are a front lens group having negative refractive power and a rear lens group having positive refracting power. In this case, to image light beams incident at a wide range of angles within an electronic imaging device in a predetermined range, it is required to make the negative refracting power of the front group strong. Especially when it is attempted to set up the front group of negative refracting power with fewer lenses so as to reduce the size of the optical system, power loads on the respective lenses increase, rendering correction of aberrations difficult. Even when, in this case, it is intended to use aspheric surfaces to enhance the effect on aberration correction, lens configuration grows more complicated, offering problems such as difficulty of lens fabrication. For light incident on the electronic imaging device, it is desired to have telecentric capability, and it is required to make sure a certain flange back so as to insert various optical filters between the electronic imaging device and the optical system, which arises a problem that the total length of the optical system grows long. For that very reason, the diameter of lenses forming the front group having negative refracting power grows large, offering an obstacle to size reductions of the optical system.
With the superwide-angle zoom lens set forth in Patent Publication 1 (JP(A)2004-21223), superwide-angle design is achieved by using a lot of aspheric surfaces while the negative front group is simplified in construction. Because the power construction of the first lens that is an aspheric surface is not tight, however, that zoom lens is still less than satisfactory in both total length reductions and diameter reductions.
With the superwide-angle zoom lens disclosed in Patent Publication 2 (JP(A)2007-94176), the correction of aberrations necessary for superwide-angle design is implemented by applying aspheric surfaces to the negative second lens. For this reason, however, the first lens grows large, failing to achieve size reductions.
With the superwide-angle zoom lens shown in Patent Publication 3 (JP(A)2001-166206), the correction of aberrations necessary for superwide-angle design is implemented by applying aspheric surfaces to the first and second lenses having negative refracting power and incorporating a plurality of positive lens components in the first lens group. To this end, however, a lot more lenses of strong power are needed, rendering the optical system bulky. In addition, that zoom lens is generally unsatisfactory in terms of how to avoid decentration and slash cost.
Having been made with the aforesaid problems in mind, the invention has for its object the provision of a small-format, superwide-angle zoom lens that is well compatible with a lens interchangeable type single-lens reflex camera through the optimization of the construction of the first group, and an imaging apparatus incorporating the same.